How exactly do you make it? Offhand, something with a little acidity would probably be a good bet, but it would be easier to troubleshoot if you detail how you make the dish beyond just listing the ingredients.

I might add chopped tomatoes (or chopped diced canned depending on your growing season) along with the thyme. But if I did that I might leave out the leeks.... kind of like a lentil soup flavor to the sauce. I don't' know if you want to change the recipe that much, but that is what came to mind.

I agree about adding acidity- or something spicy, or sweet, or umami, or some combination therein. Maybe sriracha, dried fruit, or truffle salt. A splash of sherry if you have some. I like bits of browned sausage or crispy bacon in my lentils, if you aren't vegetarian. I also think the garlic and leeks should be sauteed.